The present invention relates in general to telecommunication techniques. More particularly, the invention provides a method and system for subcarrier modulation as supervisory channel. Merely by way of example, the invention is described as it applies to optical networks, but it should be recognized that the invention has a broader range of applicability.
Telecommunication techniques have progressed through the years. As merely an example, optical networks have been used for conventional telecommunications in voice and other applications. The optical networks can transmit multiple signals of different capacities. For example, the optical networks terminate signals, multiplex signals from a lower speed to a higher speed, switch signals, and transport signals in the networks according to certain definitions.
To monitor these signals, the optical networks often track the flow of data through various nodes, switching sites and links. For example, the optical networks provide supervisory signals propagating with the data signals. The supervisory signals provide information for maintaining and monitoring the optical networks. Several conventional techniques have been proposed to transmit and receive such supervisory signals.
In Dense Wavelength Division Multiplexing (DWDM) systems, the supervisory signals are transmitted in a separate channel at a wavelength outside of the data band. For example, the supervisory channel is either at 1510 nm or 1620 nm outside the wavelength band for data signal transmissions. The data band is usually in the range of 1530 nm to 1562 nm for C-band or 1570 nm to 1610 nm for L-band. This transmission method may be a good solution if majority of the wavelength channels are deployed and the cost of the supervisory channel is shared by all data channels. But in many practical situations, especially in metro area Wavelength Division Multiplexing (WDM) applications, only a few channels are usually deployed. In these cases, the cost of a separate WDM supervisory channel becomes significant, and the solution is usually not cost effective.
Another method of carrying supervisory signals uses the Frequency Division Multiplexing (FDM) mechanism. The FDM method transmits the supervisory signals by subcarrier transmissions. The supervisory signals and the data signals are usually generated by the same transmitters and share the same wavelengths. The subcarrier frequency of the supervisory channel is outside of the bandwidth of the data signals. For example, the subcarrier frequency equals about 3 GHz for a baseband data rate up to 2.5 Gbps. To accommodate the high subcarrier frequency, the transmitters and receivers need to have a frequency response higher than one for transmitting only the data signals. Non-standard and high cost components are usually used. For example, a separate, higher speed, receiver may be used to detect the supervisory channel signal.
Similarly, the supervisory signals can be transmitted with a low frequency subcarrier or “pilot tone.” Lowering the subcarrier frequency to the kHz or MHz range reduces the cost of components and the difficulty of implementation, but the performance of the supervisor signal is usually degraded.
Hence it is highly desirable to improve techniques for transmitting and receiving supervisor signals.